The sounds of high winds

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II ACOUSTICAL PRACTICE AND SOUND RESEARCH II.1. Different points of view In 2001 the German wind farm Rhede was put into operation close to the Dutch border. Local authorities as well as residents at the Dutch side had opposed the construction of the 17 wind turbines because of the effects on landscape and environment: with 98 m hub height the 1.8 MW turbines would dominate the skyline of the early 20th century village of Bellingwolde and introduce noise in the quiet area. With the turbines in operation, residents at 500 m and more from the wind farm found the noise (and intermittent or flicker shadow, which will not be dealt with here) worse than they had expected. The wind farm operator declined to take measures as acoustic reports showed that German as well as Dutch noise limits were not exceeded. When the residents brought the case to a German court, they failed on procedural grounds. For a Dutch court they had to produce arguments that could only be provided by experts. Science Shops are specifically intended to help non-profit groups by doing research on their behalf. For the Science Shop for Physics in Groningen noise problems constitute the majority of problems that citizens, as a group or individually, come up with. Although the aim of our research is the same as for acoustic consultants –to quantify sound levels relevant for annoyance- the customers are different: consultants mostly work for the party responsible for the sound production, whereas the Science Shop mostly works for the party that is affected by the sound. This may lead to different research questions. In the case of wind farm Rhede a consultancy will check the sound production of the turbines and check compliance of the calculated sound immission level with relevant limits. However, the Science Shop, taking the strong reaction from the residents as a starting point, wanted to check whether the real sound immission agrees with the 17
calculated one and whether sound character could explain extra annoyance. In the Dutch professional journal ‘Geluid’ it was shown, on the basis of 30 acoustic reports, that acoustic consultants tend to rely too much on information from their customers, even when they had reason to be critical about it [Van den Berg 2000]. As consultants’ customers are usually noise producers and authorities, the point of view of those that are affected by noise is not usually very prominent. This book shows that for wind turbines a similar case can be made. II.2 Results from our wind turbine research The results of the investigation of the sound from the wind farm Rhede are given in the next chapters. Here the results will be dealt with briefly. The main cause for the high sound level perceived by residents is the fact that wind velocities at night can, at 100 m height, be substantially higher than expected. As a consequence a wind turbine produces more sound. As measured immission levels near the wind farm Rhede show, the discrepancy may be very large: sound levels are up to 15 dB higher than expected at 400 m from the wind farm. The important point is not so much that the maximum measured sound level is higher than the maximum expected sound level (it was, around +2 dB, but this was not an effect of the wind velocity profile). The point is that this maximum does not only occur at high wind velocities as expected, accompanied by high wind induced ambient sound levels, but already at relatively low wind velocities (4 m/s at 10 m height) when there is little wind at the surface and therefore little wind induced background sound. Thus, the discrepancy of 15 dB occurs at quiet nights, but yet with wind turbines at almost maximum power. This situation occurs quite frequently. A second effect that adds to the sound annoyance is that the sound has an impulsive character. The primary factor for this appeared to be the well known swishing sound one hears close to a turbine. For a single turbine these 1 – 2 dB broad band sound pressure fluctuations would not classify as impulsive, but at night this swish seems to evolve into a less gentle thumping. Also, when several turbines operate nearly synchronously the 18
Page 1 and 2: The sounds of high winds the effect
Page 3 and 4: Promotores: prof dr ir H. Duifhuis
Page 5 and 6: Contents I WIND POWER, SOCIETY, THI
Page 7 and 8: VII THINKING OF SOLUTIONS: measures
Page 10 and 11: I WIND POWER, SOCIETY, THIS BOOK: a
Page 12 and 13: esidents) noticed the discrepancy b
Page 14 and 15: that even though wind turbines did
Page 16 and 17: legitimate problems [Wolsink 1990].
Page 18 and 19: Thinking that this could perhaps be
Page 20 and 21: gas must keep flowing” . 1 I do n
Page 22 and 23: sound and the noise that wind produ
Page 24: combined with the corresponding sec
Page 29 and 30: annoying turbine sound at distances
Page 31 and 32: was 57.9 dB, with a maximum deviati
Page 33 and 34: not correct at night, although the
Page 36 and 37: III BASIC FACTS: wind power and the
Page 38 and 39: 0.1. In a stable atmosphere vertica
Page 40 and 41: With regard to wind power some atte
Page 42 and 43: III.4 Main sources of wind turbine
Page 44 and 45: High frequency: trailing edge (TE)
Page 46: So, what's the sound like...? (....
Page 49 and 50: Figure IV.2: turbines (dots W1….W
Page 51 and 52: L Wj , assumed identical for all k
Page 53 and 54: IV.1. As explained above, the wind
Page 55 and 56: sound level in the measurement inte
Page 57 and 58: Leq,5 min in dB(A) 50 45 40 35 30 A
Page 59 and 60: The same lines as in figure IV.5B,
Page 61 and 62: at hub height, not to a variation i
Page 63 and 64: IV.10.1 Measured and calculated imm
Page 65 and 66: wind farm to the measurement locati
Page 67 and 68: spherical divergence, that is: prop
Page 69 and 70: Thus, the logarithmic wind profile,
Page 71 and 72: There are now three factors influen
Page 73 and 74: direction may change significantly.
Page 75 and 76:
V.2 Measurement results V.2.1 Locat
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P48 microphone. The sound was then
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1/3 octave band Lp in dB 1/3 octave
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1 1 0 1 0 0 1 0 0 0 1 0 0 0 0 A B 8
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same magnitude as close to the turb
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In figure V.5 the equivalent 1/3 oc
Page 87 and 88:
Also plotted in figure V.7 is the v
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V.3 Perception of wind turbine soun
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kHz. For wind turbines we found tha
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V.4 Conclusion Atmospheric stabilit
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VI STRONG WINDS BLOW UPON TALL TURB
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(hub height) wind velocity is 4 m/s
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For the hourly progress of wind vel
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In figure VI.5 the frequency distri
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when V 80 was over 4 m/s. Such a de
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dunes on the North Sea coast, Vliss
Page 108 and 109:
when based on extrapolated wind vel
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The differences between actual and
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hours. This corresponds to an avara
Page 114 and 115:
VII THINKING OF SOLUTIONS: measures
Page 116 and 117:
stability. The turbine thus operate
Page 118 and 119:
sound level was measured close to a
Page 120 and 121:
This type of control can also be ac
Page 122 and 123:
consecutive periods of 15 minutes i
Page 124 and 125:
increase of blade pitch with tilt (
Page 126 and 127:
elatively quiet areas as it control
Page 128 and 129:
VIII RUMBLING WIND: wind induced so
Page 130 and 131:
Finally, in this overview, Boersma
Page 132 and 133:
The friction created by wind shear
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the eddy relative to the screen sur
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(L at,1/3 ~ -26.7·logf), but press
Page 138 and 139:
VIII.3 Comparison with experimental
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the small size of the unscreened mi
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A B reduced pressure level Lred (dB
Page 144 and 145:
microphone, in a 9 cm foam cylinder
Page 146 and 147:
plotted is the calculated screening
Page 148 and 149:
VIII.5 Applications As microphone w
Page 150 and 151:
IX GENERAL CONCLUSIONS The research
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modulation frequencies close to 4 H
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Results from various onshore, relat
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limits, but nighttime immission lev
Page 158 and 159:
X EPILOGUE This is the end of my to
Page 160 and 161:
“….. about 80 per cent of the p
Page 162 and 163:
ACKNOWLEDGMENTS I want to express m
Page 164 and 165:
SUMMARY Ch. III Ch. II Ch. I This s
Page 166 and 167:
and as a result layers of air are l
Page 168 and 169:
500 kW. However, based on the real,
Page 170 and 171:
well known for an unobstructed wind
Page 172 and 173:
SAMENVATTING Bobby vraagt: 'Hoort u
Page 174 and 175:
Vaak wordt aangenomen dat er een va
Page 176 and 177:
gebleken dat de menselijke gevoelig
Page 178 and 179:
Bij een windpark kunnen de fluctuat
Page 180 and 181:
REFERENCES Archer C.L. and Jacobson
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Kerkers A.J. (1999): “Windpark Rh
Page 184 and 185:
Petersen E.L., Mortensen N.G., Land
Page 186 and 187:
Van Ulden, A.P. and Wieringa J. (19
Page 188 and 189:
APPENDICES 179
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dB(G): unit of level after G-weight
Page 192 and 193:
u f : rms longitudinal component of
Page 194 and 195:
downwind turbine with an 80 m tubul
Page 196 and 197:
and total trailing edge immission s
Page 198 and 199:
So for a modern turbine at high spe
Page 200 and 201:
45 wind E (100 gr) 20 Leq (A) Leq,5
Page 202 and 203:
Wiens brood men eet, ….. Een plei
Page 204 and 205:
Vochtproblemen in woningen Venuslaa
Page 206 and 207:
Geluidsbelasting van een windturbin
Page 208 and 209:
Metingen van laagfrequent geluid in
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